High resolution resistivity earth imager

ABSTRACT

An imaging tool made includes a pad whose curvature is chosen based on the expected range of borehole radius and the pad size so as to maintain the maximum standoff below a desired value. The curvature may be adjusted using fasteners.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/199,278 filed Aug. 27, 2008 the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

This disclosure generally relates to exploration for hydrocarbonsinvolving electrical investigations of a borehole penetrating an earthformation. More specifically, this disclosure relates to a method andapparatus having the flexibility to provide an image of a borehole wallfor a wide range of borehole sizes.

2. Background of the Art

Electrical borehole logging is often used to provide images of anelectrical property of boreholes. There are two categories of devicestypically used as electrical logging devices for imaging boreholes. Inthe first category (galvanic devices), a measure electrode (currentsource or sink) is used in conjunction with a return electrode (such asthe tool body). A current flows from a current source in the tool to ameasure electrode through the earth formation. The current returns tothe source via another electrode. The second category relates toinductive measuring tools in which an antenna within the electricallogging tool induces a current flow within the earth formation. Themagnitude of the induced current is detected using either the sameantenna or a separate antenna.

There are several modes of operation of a galvanic device. In one mode,the current at a current electrode is maintained constant and a voltageis measured between a pair of monitor electrodes. In another mode, thevoltage of the measure electrode is fixed and the current flowing fromthe measure electrode is measured.

The galvanic devices are contact devices, in that the measure electrodesgenerally come in contact with the borehole wall during logging of thewellbore. Such devices are sensitive to the effects of boreholerugosity, borehole size, and the standoff of the pad carrying themeasure electrodes. The present disclosure provides an improvedelectrical logging tool that reduces the effects of the pad offset.

SUMMARY OF THE DISCLOSURE

One embodiment of the disclosure provides a method of generating animage of a resistivity property of an earth formation. In one aspect, amethod may include: determining a size of a borehole in which aresistivity imaging instrument is to be used; selecting a radius ofcurvature of a pad of the resistivity imaging instrument based on a padsize, the size of the borehole, and a selected value for a maximumstandoff of the pad; conveying the resistivity imaging instrument intothe borehole; using a plurality of sensors on the pad responsive to aformation resistivity to obtain measurements indicative of theresistivity property; and providing the image of the resistivityproperty of the earth formation using the measurements obtained by theplurality of sensors.

Another embodiment of the disclosure includes an apparatus for providingan image of a resistivity property of an earth formation. In one aspect,the apparatus may include a resistivity imaging instrument configured tobe conveyed into a borehole, wherein the he resistivity imaginginstrument includes a pad having a radius of curvature that isdetermined by using a pad size, a borehole size, and a selected valuefor a maximum standoff of the pad between the pad and inside of theborehole. The apparatus may further include at least one processorconfigured to: use an output of each of a plurality of sensors on thepad responsive to a resistivity property of the earth formation toprovide an image of the resistivity property.

Another embodiment according the disclosure provides acomputer-readable-medium for use with an apparatus for providing aresistivity image of an earth formation, wherein the apparatus comprisesa resistivity imaging instrument configured to be conveyed into aborehole, the resistivity imaging instrument having a pad with a radiusof curvature determined using a pad size, a borehole size, and a definedvalue for a maximum standoff of the pad; wherein thecomputer-readable-medium comprises instructions that enable at least oneprocessor to: use an output of each of a plurality of sensors on the padresponsive to a resistivity property of the earth formation to providean image of the resistivity property.

BRIEF DESCRIPTION OF THE FIGURES

The present disclosure is best understood with reference to theaccompanying figures in which like numerals generally refer to likeelements and in which:

FIG. 1 shows an exemplary logging tool that includes an imaging toolmade according to one embodiment of the disclosure in a borehole.

FIG. 2A is a schematic diagram of an exemplary imaging tool madeaccording to one embodiment of the disclosure for use in a logging tool,such as the logging tool of FIG. 1;

FIG. 2B is a detailed view of a pad made according to one embodiment ofthe disclosure for use in an imaging tool, such as the imaging toolshown in FIG. 2A;

FIG. 3A illustrates a pad having a smaller radius of curvature than aborehole;

FIG. 3B illustrates a pad having a larger radius of curvature than aborehole;

FIG. 4 shows a relation between pad radius, borehole radius and maximumstandoff according to one aspect of the disclosure for use in an imagingtool, such as the imaging tool shown in FIG. 2;

FIG. 5 shows a pad for which the radius of curvature may be adjusted;and

FIG. 6 shows an exemplary image obtained with a resistivity imagingtool.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 shows an exemplary logging tool 10 suspended in a borehole 12that penetrates an earth formations 13 from a suitable cable 14 thatpasses over a sheave 16 mounted on drilling rig 18. By industrystandard, the cable 14 includes a stress member and seven conductors fortransmitting commands to the tool and for receiving data back from thetool as well as power for the tool. The tool 10 is raised and lowered bydraw works 20. Electronic module 22, on the surface 23, transmits therequired operating commands downhole and in return, receives data backwhich may be recorded on an archival storage medium of any desired typefor concurrent or later processing. The data may be transmitted inanalog or digital form. Data processors such as a suitable computer 24,may be provided for performing data analysis in the field in real timeor the recorded data may be sent to a processing center or both for postprocessing of the data. The tool 10 includes a novel pad that isdiscussed below.

FIG. 2A is a schematic external view of a borehole imaging tool 10 madeaccording to one embodiment of the disclosure. The tool 10 comprisesresistivity arrays 26 and, optionally, a mud cell 30 and acircumferential acoustic televiewer 32. Electronics modules 28 may belocated at suitable locations in the system and not necessarily in thelocation indicated. The various components of the imaging tool may bemounted on a mandrel 34 in any suitable manner. An orientation module 36including a magnetometer and an accelerometer or inertial guidancesystem may be mounted above the imaging assemblies 26 and 32. The upperportion of the tool 10 contains a telemetry module 38 for sampling,digitizing and transmission of the data samples from the variouscomponents of the tool 10 to the surface electronic module 22. The datafrom the various components is digitized before sending such data to thesurface electronic module 22. In an alternative arrangement, the datamay be transmitted in analog form and digitized by surface electronics22.

Also shown in FIG. 2A are three resistivity arrays 26 (a fourthresistivity array is hidden in this view). Referring to FIGS. 2A and 2B,each pad 40 placed on a spring member 42. A pad 42 includes an array ofmeasure electrodes 41 a, 41 b, . . . 41 n for injecting electricalcurrents into the formation, focusing electrodes 43 a, 43 b forhorizontal focusing of the electrical currents from the measureelectrodes and focusing electrodes 45 a, 45 b for vertical focusing ofthe electrical currents from the measure electrodes. For the purpose ofthis disclosure the term, “vertical” refers to the direction along theaxis of the borehole and “horizontal” refers to a plane perpendicular tothe vertical.

FIG. 3A shows an exemplary pad 303 having a radius of curvature 305inside a borehole 301. In the pad shown, the radius of curvature of thepad 303 is less than the radius of the borehole, so that at the centerof the pad the standoff is zero and it increases away from the center ofthe pad.

In contrast, in FIG. 3B, the pad 303′ has a larger radius of curvature305′ than the radius of the borehole 301, so that the center of the padhas a standoff from the borehole wall while the edges of the pad have asmaller or no standoff. The results of calculations showing the effectsof pad radius and borehole radius are discussed reference to FIG. 4.

Shown in FIG. 4 are plots of the pad radius of curvature (in inches) asthe asbscissa and the standoff (ordinate) for borehole radius of 3.00″(7.62 cm) 401, 3.25″ (8.255 cm) 403, 4″ (10.16 cm) 405, 5″ (12.7 cm)407, 6″ (15.48 cm) 409, 7″ (17.78 cm) 411 and 8″ (20.32 cm) 413. Theplots correspond to the maximum standoff for the exemplary pad.

In FIG. 4, the curves with negative slope correspond to cases where theresistivity electrodes on the edge of the pad have the maximum standoffwhile the curves with positive slope correspond to the case where themeasure electrodes at the center of the pad have the maximum standoff.

FIG. 4 is instructive in that for the exemplary tool a pad radius ofcurvature of between 4.2″ (10.67 cm) and 4.6″ (11.69 cm) defined by theregion 421 has a maximum acceptable standoff of less than ¼″ (6.35 mm)that has been found to be acceptable in simulations and in practice.This is acceptable for a large range of borehole sizes (from 6″ diameterto 16″ diameter. This leads to one embodiment of the disclosure, namelyselecting a pad having a radius of curvature that has an acceptablestandoff (less than a specified maximum) for the specified range ofborehole sizes to be encountered in practice. The pad may be configuredfor any suitable standoff.

The results of FIG. 4, as an example, also lead to a second embodimentof the disclosure, illustrated in FIG. 5. Shown in FIG. 5 is a pad 303″.That includes a backing material 501 and a set of adjustable fasteners503 a, 503 b, 503 c, which can be adjusted to change the radius ofcurvature of the pad face. In this example, three fasteners are shown,but this is not to be construed as a limitation to the disclosure. Thoseversed in the art would recognize that a minimum of a single fastener inthe center is sufficient to alter the curvature of the pad 303, while aplurality of fasteners will typically provide a better match of the padface to the borehole wall.

The electronics associated with each pad may be the same as in priorart. Any suitable coupling arrangement including but not limited tothose that can withstand the pressure and fluids downhole may be used.The present disclosure does envisage the possibility of using fewer thanthe full complement of electrodes on the pad. In such cases, the sameazimuthal electrode spacing may correspond to a smaller azimuthalseparation for a borehole of large radius than for a borehole of asmaller radius. Using a subset of the azimuthal electrodes providesflexibility in the azimuthal resolution of the produced image. Using asubset of the electrodes may also be used to reduce the effectiveazimuthal pad size when the standoff on the edges of the pad isexcessive.

For a borehole of larger radius, there may be gaps in the full 360°image since each pad would provide a smaller azimuthal coverage. Anysuitable interpolation technique may be used to fill in the gaps in theimage.

Referring now to FIG. 6, an exemplary image is shown. A gamma ray log isshown by 603, a two dimensional (2-D) image of the borehole wall with afixed gain display by 605 and a 2-D image of the borehole wall with adynamic gain applied to the display by 607. Two isometric views of theborehole wall in cylindrical geometry are shown in 609. The determinedimage may then be recorded on a suitable medium such as a memory device.

The embodiments presented herein relate to galvanic devices in which thecurrent in current electrodes is used for resistivity imaging. Theprinciples and methods described above may also be used for galvanicdevices in which voltages across pairs of electrodes are used. Theprinciples and method described above are equally applicable toinduction devices in which small antenna loops are used to measure themagnetic field produced by induced currents flowing in the formation.For this reason, the term sensor is intended to include both electrodesand sensing antennas.

The description above has been in the context of a wireline conveyedimaging pad. An implementation for a measurement-while-drilling is alsopossible. Drilling is carried out by a drillbit on a bottomhole assembly(BHA) conveyed on a drilling tubular. In particular, there are at leasttwo situations in which the pad curvature may need to be adjusted tomatch the curvature of the borehole. The first situation where a padmounted imaging device may be used is when drilling is done using anoversized drillbit. The use of an extendable pad has been discussed, forexample, in U.S. Pat. No. 5,242,020 to Cobern, having the same assigneeas the present disclosure and the contents of which are incorporatedherein by reference. The second situation is when there is an axialseparation between the drillbit and the imaging pad and the borehole hascaved in or been washed out.

Implicit in the processing of the data is the use of a computer programimplemented on a suitable machine readable medium that enables one ormore processors to perform the acquisition and processing. The termprocessor as used in this application is intended to include suchdevices as field programmable gate arrays (FPGAs). The machine readablemedium may include ROMs, EPROMs, EAROMs, Flash Memories and Opticaldisks. As noted above, the processing may be done downhole or at thesurface.

While the foregoing disclosure is directed to certain embodiments of thedisclosure, various modifications will be apparent to those skilled inthe art. It is intended that all such variations within the scope andspirit of the appended claims be embraced by the foregoing disclosure.

1. A method of estimating a resistivity property of an earth formation,the method comprising: determining a size of a borehole in which aresistivity imaging instrument is to be used, the resistivity imaginginstrument including a pad having a face configured to have anadjustable radius of curvature; adjusting the radius of curvature of thepad face using at least the determined borehole size; conveying theresistivity imaging instrument into the borehole; measuring a parameterindicative of the resistivity property using a plurality of sensors onthe pad; and imaging the resistivity property using the measurementsobtained by the plurality of sensors.
 2. The method of claim 1, whereinthe radius of curvature is adjusted by also using a selected value for amaximum standoff of the pad.
 3. The method of claim 2 further comprisingspecifying the value of maximum standoff based on a simulation result.4. The method of claim 1 wherein the plurality of sensors compriseelectrodes and the measurements comprise currents.
 5. The method ofclaim 1 wherein the plurality of sensors comprise antennas responsive toa magnetic field produced by a current in the formation.
 6. The methodof claim 1 further comprising conveying the resistivity imaginginstrument into the borehole as part of a string of logging instrumentson a wireline.
 7. The method of claim 1 wherein the imaging is performedusing an output of a subset of the plurality of sensors.
 8. An apparatusfor providing an image of a resistivity property of an earth formation,the apparatus comprising: a resistivity imaging instrument including: apad, the pad including a face having an adjustable radius of curvature;a plurality of sensors responsive to a resistivity property of the earthformation; and at least one processor configured to use an output ofeach of plurality of sensors on the pad to provide an image of theresistivity property.
 9. The apparatus of claim 8, wherein theadjustable radius of curvature of the pad reduces a stand off at one of:(i) a center of the pad, and (ii) a location away from the center of thepad.
 10. The apparatus of claim 8, wherein the radius of curvature isadjusted at one of: (i) a single location on the pad face, and (ii) aplurality of locations on the pad face.
 11. The apparatus of claim 8wherein the plurality of sensors comprise electrodes and the output ofthe sensors comprise currents.
 12. The apparatus of claim 8 wherein theplurality of sensors comprise antennas responsive to a magnetic fieldproduced by a current in the formation.
 13. The apparatus of claim 8further comprising a wireline configured to convey the resistivityimaging instrument into the borehole.
 14. The apparatus of claim 8further wherein the pad is extendable from a bottomhole assemblyconveyed on a drilling tubular.
 15. A non-transitory computer-readablemedium product accessible to a processor, the computer-readable mediumincluding instructions which enable the processor to use measurementsmade by a resistivity imaging instrument to image a resistivity propertyof a formation, the resistivity imaging instrument including a padhaving a face with an adjustable radius of curvature and a plurality ofsensors responsive to the resistivity property of the earth formation.16. The non-transitory computer-readable medium product of claim 15further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) anEAROM, (iv) a flash memory, and (v) an optical disk.